Systems and methods for producing an image from a rotational intravascular ultrasound device

ABSTRACT

The invention generally relates systems and methods to for producing an image from a rotational intravascular ultrasound device. A method can include alternately transmitting complementary Golay codes to a plurality of transducers in a intravascular ultrasound device; receiving echoes of the complementary codes from the transducers; performing pulse compression of the echoes that comprises weighting the received echoes and summing an odd number of weighted echoes, wherein a center echo is given a weighted value of 1.0 and weighted sums of its neighbors constitute complementary echoes of a Golay pair; and producing an image from the compressed echoes. A system can include a processor; and a plurality of beam modules coupled to the processor, each module comprising: a receiver for receiving a trigger signal from the processor; a complex programmable logic device programmed with a Golay code; a high voltage switching transmitter; and an ultrasound transducer.

RELATED APPLICATION

The present application claims the benefit of and priority to U.S.provisional patent application Ser. No. 61/778,757, filed Mar. 13, 2013,the content of which is incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The invention generally relates to systems and methods to for producingan image from a rotational intravascular ultrasound device.

BACKGROUND

In ultrasound imaging, spatial resolution and depth penetration areimportant parameters to quantitatively assess image quality. Generally,the wavelength at the ultrasound center frequency determines spatialresolution, with revolution improving as the frequency increases.However, tissue attenuation typically increases as a linear function offrequency, even becoming nonlinear at higher frequencies (e.g., greaterthan 40 MHz) commonly employed in intravascular ultrasound imagingapplications. Although high frequency ultrasound facilitates highresolution imaging in the near field, the depth penetration may becompromised, rendering the deep tissue structures difficult to identify.

To enhance the depth penetration without the loss of spatial resolution,transmit voltage levels are typically increased to deliver more transmitenergy to the imaging target. However, this approach is limited both byregulations of diagnostic medical ultrasound and by the nature of theintravascular ultrasound imaging environment, such as the long cablelength between ultrasound transducer elements and imaging systemelectronics and the small size of electronics integrated with thecatheter mounted transducer.

Rather than increasing the transmit voltage levels, coded excitationmethods using elongated modulated transmit bursts have been employed toaddress the dilemma of maintaining depth of penetration while increasingoperating frequency for improved spatial resolution. This method iscalled pulse compression. An ultrasound imaging apparatus using pulsecompression employs a coded long pulse instead of the conventional shortpulse. One type of coded excitation methods uses Golay codes. A Golaycode is a binary code modulated with a short burst. Two different binarycode (of particular sequence) constitute a Golay pair. When two codesare separately decoded and summed, range sidelobes are completelyeliminated with only the main lobe remaining. Due to that characteristicof Golay codes, there have been great endeavors to take advantage ofGolay codes in ultrasound imaging apparatuses.

In practice, the range sidelobes are not completely removed, due tononlinear propagation of ultrasound within tissue structures, motionartifacts, and other non-idealities. Particularly for rotationalintravascular ultrasound, the continuous rotation causes a slightangular misalignment for adjacent A-scans used to produce a Golay pair,resulting in increased range sidelobe levels.

SUMMARY

The invention uses short burst modulated Golay codes with a multi-beamapproach to suppress the motion artifact in Golay code rotationalintravascular ultrasound (IVUS). The multi-beam strategy for use withGolay codes involves alternately transmitting complementary Golay codesand then summing an odd number (greater than one) of weighted, decodedA-scans.

In certain aspects, the invention provides methods for producing animage from a rotational intravascular ultrasound device. The methodsinvolve alternately transmitting complementary Golay codes to aplurality of transducers in an intravascular ultrasound device.Additionally, the methods involve receiving echoes of the complementarycodes from the transducers and performing pulse compression of theechoes. The pulse compression involves weighting the received echoes andsumming an odd number of weighted echoes, in which a center echo isgiven a weighted value of 1.0 and weighted sums of its neighborsconstitute complementary echoes of a Golay pair. An image is producedfrom the compressed echoes, and that image may be displayed. Methods ofthe invention may additionally involve computing convolution of thereceived echoes of the complementary codes.

Any technique known in the art may be used to produce the Golay codes.In certain embodiments, the Golay codes are produced by applying abi-phase window over the Golay codes. The bi-phase window may be one ofa bi-phase rectangular window, a bi-phase Hamming window, a bi-phaseHanning window and bi-phase Bartlett window.

Another aspect of the invention provides a system for producing anultrasound image from a rotational intravascular ultrasound device. Thesystem includes a processor, and a plurality of beam modules coupled tothe processor. Each module includes a receiver for receiving a triggersignal from the processor, a complex programmable logic deviceprogrammed with a Golay code, a high voltage switching transmitter, andan ultrasound transducer.

The processor alternately transmits trigger signals to the beam modules,thereby causing the beam modules to alternately transmit complementaryGolay codes. The processor receives echoes of the complementary codes.The processor performs pulse compression of the echoes that includesweighting the received echoes and summing an odd number of weightedechoes, in which a center echo is given a weighted value of 1.0 andweighted sums of its neighbors constitute complementary echoes of aGolay pair. The processor produces an image from the compressed echoes.The processor is coupled to a display device and causes the image to bedisplayed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a set of graphs showing the principle of side lobecancellation using pair of Golay complementary sequences of length 8.

FIG. 2 illustrate a system of the invention.

FIG. 3 is a diagraph showing how methods of the invention are performed.

DETAILED DESCRIPTION

The invention generally relates systems and methods to for producing animage from a rotational intravascular ultrasound (IVUS) device. Systemsand methods of then invention are particularly useful for rotationalIVUS. In a rotational IVUS catheter, a single transducer having apiezoelectric crystal is rapidly rotated (e.g., at approximately 1800revolutions per minute) while the transducer is intermittently excitedwith an electrical pulse. The excitation pulse causes the transducer tovibrate, sending out a series of transmit pulses. The transmit pulsesare sent at a frequency that allows time for receipt of echo signals.The sequence of transmit pulses interspersed with receipt signalsprovides the ultrasound data required to reconstruct a completecross-sectional image of a vessel.

The general design and construction of rotational IVUS catheters isshown, for example in Yock, U.S. Pat. Nos. 4,794,931, 5,000,185, and5,313,949; Sieben et al., U.S. Pat. Nos. 5,243,988, and 5,353,798;Crowley et al., U.S. Pat. No. 4,951,677; Pomeranz, U.S. Pat. No.5,095,911, Griffith et al., U.S. Pat. No. 4,841,977, Maroney et al.,U.S. Pat. No. 5,373,849, Born et al., U.S. Pat. No. 5,176,141, Lancee etal., U.S. Pat. No. 5,240,003, Lancee et al., U.S. Pat. No. 5,375,602,Gardineer et at., U.S. Pat. No. 5,373,845, Seward et al., Mayo ClinicProceedings 71(7):629-635 (1996), Packer et al., Cardiostim Conference833 (1994), “Ultrasound Cardioscopy,” Eur. J.C.P.E. 4(2):193 (June1994), Eberle et al., U.S. Pat. No. 5,453,575, Eberle et al., U.S. Pat.No. 5,368,037, Eberle et at., U.S. Pat. No. 5,183,048, Eberle et al.,U.S. Pat. No. 5,167,233, Eberle et at., U.S. Pat. No. 4,917,097, Eberleet at., U.S. Pat. No. 5,135,486, and other references well known in theart relating to intraluminal ultrasound devices and modalities. Thecatheter will typically have proximal and distal regions, and willinclude an imaging tip located in the distal region. Such catheters havean ability to obtain echographic images of the area surrounding theimaging tip when located in a region of interest inside the body of apatient. The catheter, and its associated electronic circuitry, willalso be capable of defining the position of the catheter axis withrespect to each echographic data set obtained in the region of interest.

Systems and methods of the invention use Golay codes. Use of Golay codesin ultrasound is described for example in U.S. Pat. Nos. 7,535,797;6,958,042; 6,663,565; 6,638,227; 6,491,631; 6,375,618; 6,350,240;6,312,384; 6,210,332; 6,186,949; and 6,146,328, the content of each ofwhich is incorporated by reference herein in its entirety.

Golay complementary sequences are pairs of binary codes, belonging to abigger family of signals called complementary pairs, which consist oftwo codes of the same length N whose auto-correlation functions haveside-lobes equal in magnitude but opposite in sign. Summing them upresults in a composite auto-correlation function with a peak of 2N andzero side-lobes. FIG. 1 illustrates the principle of theside-lobe-canceling for a pair of signed of length equal to 8 bits each.

There are essentially several algorithms for generating Golay pairs. Letthe variables a_(i) and b_(i)(i=1, 2, . . . n) are the elements of twon-long complementary series equal either ‘+1’ or ‘−1’, [3],A=a₁, a₂, . . . , a_(n);B=b₁, b₂, . . . , b_(n).   (1)

The ordered pair (A;B) are Golay sequences of length n if and only iftheir associated polynomialsA(x)=a ₁ +a ₂ x+ . . . +a _(n) x ^(n-1),B(x)=b ₁ +b ₂ x+ . . . +b _(n) x ^(n-1),   (2)satisfy the identityA(x)A(x ⁻¹)+B(x)B(x ⁻¹)=2n   (3)in the Laurent polynomial ring Z[x, x⁻¹].

Let their respectable auto-correlation functions NA and NB of thesequences A and B respectively be defined by

$\begin{matrix}{{{N_{A}(j)} = {\sum\limits_{i \in Z}\;{a_{i}a_{i + j}}}}{{N_{B}(j)} = {\sum\limits_{i \in Z}\;{b_{i}b_{i + j}}}}} & (4)\end{matrix}$where we set a_(k)=0 if k∉(1 . . . n). Now condition (3) can beexpressed by the sum N_(A)+N_(B), and

$\begin{matrix}{{{N_{A}(j)} + {N_{B}(j)}} = \left\{ \begin{matrix}{{2\; N},} & {j = 0} \\{0,} & {j\bumpeq\not{}0}\end{matrix} \right.} & (5)\end{matrix}$The sum of both autocorrelation function is at j=0 and zeroingotherwise.

The second, recursive method for constructing the Golay's sequences ispresented below. Let the variables a(i) and b(i) be the elements (i=0,1, 2, . . . 2^(n)−1) of two complementary sequences with elements +1 and−1 of length 2^(n)a ₀(i)=δ(i)b ₀(i)=δ(i)   (6)a _(n)(i)=a _(n-1)(i)+b _(n-1)(i−2^(n-1))b _(n)(i)=a _(n-1)(i)−b _(n-1)(i−2^(n-1))   (7)where δ(i) is the Kronecker delta function.

Equation (7) shows that in each step the new elements of the sequencesare produced by concatenation of elements a_(n)(i) and b_(n)(i) of thelength n.

Example:

Let n=1, then i takes values 0 and 1.a ₁(0)=a ₀(0)+b ₀(−1)=1;b ₁(0)=a ₀(0)−b ₀(−1)=1;a ₁(1)=a ₀(1)+b ₀(0)=1;b ₁(1)=a ₀(1)−b ₀(0)=−1.

As final results we obtain two complementary sequences of the length2^(n):a₁={1, 1};b₁={1, −1}.Once these operations are performed recursively for n=2, 3, 4 . . . thefollowing complementary sequences are obtained:a₂={1, 1, 1, −1};b₂={1, 1, −1, 1}.a₃={1, 1, 1, −1, 1, 1, −1, 1};b₃={1, 1, 1, −1, −1, −1, 1, −1}.a₄={1, 1, 1, −1, 1, 1, −1, 1, 1, 1, 1, −1, −1, −1, 1, −1};b₄={1, 1, 1, −1, 1, 1, −1, 1, −1, −1, −1, 1, 1, 1, −1, 1}.Similar method of generating the complementary code pairs, differingonly in the applied mathematical formalism has been described byMendieta and al. (Complementary sequence correlations with applicationsto reflectometry studies, Instrumentation and Development, 3, 6, 1996),the content of which is incorporated by reference herein in itsentirety.

FIGS. 2-3 illustrate systems and methods of the invention. The systemincludes a processor, and a plurality of beam modules coupled to theprocessor. Each module includes a receiver for receiving a triggersignal from the processor, a complex programmable logic deviceprogrammed with a Golay code, a high voltage switching transmitter, andan ultrasound transducer.

The processor alternately transmits trigger signals to the beam modules,thereby causing the beam modules to alternately transmit complementaryGolay codes. In the first instance of ultrasound transmission, oddtransducers of the transducer array transmit ultrasound pulse signalscorresponding to first code sequence (Golay code 1 (G1)). Eventransducers the transducer array transmit ultrasound pulse signalscorresponding to the second code sequence (Golay code 2 (G2)).Transmission of ultrasound pulse signals to a target object, such as ahuman body, and reception of reflected signals from the target objectoccur simultaneously. Switching between even transducers and oddtransducers of the transducer array with respect to corresponding Golaycodes in the first and second ultrasound transmissions reduces thegrating lobes. The grating lobe is the peak of a beam pattern generatedwhen the ultrasound signals is supplemented in an unwanted way.

The processor receives echoes of the complementary codes. The processorperforms pulse compression of the echoes that includes weighting thereceived echoes and summing an odd number of weighted echoes, in which acenter echo is given a weighted value of 1.0 and weighted sums of itsneighbors constitute complementary echoes of a Golay pair. For example,FIG. 3 shows that three beams are used for generating one compositeGolay coded excitation scan line. The figure shows that each transmitsignal is paired with a member of a Golay code. The system is set-upsuch that the pairs alternate (G1, G2, G1, G2, G1, G2 etc). Looking atthe top three beams as an example. The first scan line is produced bysumming one member of the pair from Golay code 2 and both members of thepair of Golay code 1. A member of the pair of Golay code 2 is the centerline with the pairs of Golay code 1 adjacent to the member of the pairof Golay code 2. The weighting for the center beam (Golay code 2) is1.0, while the weights for the neighboring beams (Goley code pair G1 andG1) are each 0.5. In this manner, the pair from Goley code 1 cancel eachother, leaving only center line beam of the member of Goley code 2,thereby removing sidelobes for scanline 1 and eliminating motionartifacts for that scan line.

The process is then repeated for each scan line and image is assembledfrom the scan lines. The processor is coupled to a display device andcauses the image to be displayed.

INCORPORATION BY REFERENCE

References and citations to other documents, such as patents, patentapplications, patent publications, journals, books, papers, webcontents, have been made throughout this disclosure. All such documentsare hereby incorporated herein by reference in their entirety for allpurposes.

EQUIVALENTS

Various modifications of the invention and many further embodimentsthereof, in addition to those shown and described herein, will becomeapparent to those skilled in the art from the full contents of thisdocument, including references to the scientific and patent literaturecited herein. The subject matter herein contains important information,exemplification and guidance that can be adapted to the practice of thisinvention in its various embodiments and equivalents thereof.

What is claimed is:
 1. A system for intravascular ultrasound (IVUS)imaging, the system comprising: a rotational IVUS catheter configured tobe positioned within a vessel of a patient, the rotational IVUS cathetercomprising a single ultrasound transducer configured to rotate about anaxis to obtain an IVUS image of the vessel; and a processor incommunication with the single ultrasound transducer, the processorconfigured to: alternately transmit a first code and a second code of apair of complementary Golay codes to the single ultrasound transducerwhile the single ultrasound transducer is rotating about the axis withinthe vessel; receive, from the single ultrasound transducer, echo signalsassociated with the pair of complementary Golay codes, the received echosignals comprising a first echo signal, a second echo signal, and athird echo signal; perform pulse compression of the received echosignals, wherein the pulse compression comprises weighting the receivedecho signals and summing an odd number of weighted echo signals, whereinthe second echo signal is associated with the second code, and whereinthe first and the third echo signals neighboring the second echo signalare each associated with the first code; and produce the IVUS image fromthe compressed echo signals.
 2. The system of claim 1, wherein thesecond echo signal comprises a weighted value of 1.0, and wherein thefirst and the third echo signals individually comprise a weighted valueless than 1.0 such that a total weighted value of the first and thethird echo signals is 1.0.
 3. The system of claim 2, wherein theweighted values of the first and the third echo signals are equal. 4.The system of claim 1, wherein a bi-phase window is applied over thepair of complementary Golay codes.
 5. The system of claim 4, wherein thebi-phase window is one of a bi-phase rectangular window, a bi-phaseHamming window, a bi-phase Hanning window, or a bi-phase Bartlettwindow.
 6. The system of claim 1, wherein the processor is configured tocompute a convolution of the received echo signals of the pair ofcomplementary Golay codes.
 7. The system of claim 1, wherein theprocessor is coupled to a display device and causes the IVU S image tobe displayed.
 8. The system of claim 1, wherein the pulse compressionfurther comprises: summing the first echo signal, the second echosignal, and the third echo signal to generate a first scanline; andsumming the second echo signal, the third echo signal, and a fourth echosignal to generate a second scanline, wherein the fourth echo signal isassociated with the second code.
 9. The system of claim 8, wherein: forthe first scanline, the second echo signal is given a weighted value of1.0 and the third echo signal is given a weighted value of less than1.0, and for the second scanline, the second echo signal is given aweighted value of less than 1.0, and the third echo signal is given aweighted value of 1.0.
 10. The system of claim 1, wherein the processoris configured to receive: the first echo signal at a first time; thesecond echo signal at a second time; and the third echo signal at athird time.